Another user can't act on the app you are running and a script has to be fired by a script trigger, it can't be fired by itself. Maybe there is the OnTimer script trigger is set. If you have FileMaker Pro Advanced, you may have a look at the script debugger.

What version of FMP/FMPA are you running, on what OS and version of that OS ?

Posting a small video showing the issue may help us finding what's wrong.